The present invention relates to a gamma correction circuit for a single-plate type video camera having a charged-coupled device (CCD) image pick-up device using complementary color filtering method, and more particularly, to a gamma correction circuit positioned between a correlating double-sampling device and a color signal separating circuit.
Picture tube shows a color reproduction in which the light beam does not vary in proportion to the image signal. A video camera for generating an image signal should take this characteristic of the picture tube into the consideration and, accordingly, correct the picture signal inversely to the non-linear color reproduction, which is called gamma correction. The non-linear color reproduction is called the gamma characteristic of the camera.
The conventional single-plate type video camera separates the color line sequence signal generated from the CCD image pick-up elements into primary colors by means of a color separation circuit, and then performs the gamma correction to the thus-separated primary color signals. That is, respective primary color signals (R, G and B) and the luminance signal need to be gamma-corrected separately. Thus the overall circuit becomes complex.
Especially, video cameras utilizing digital signal processing are restricted in that the resolution of the parts constituting the gamma-correction circuit is higher than those of other circuits.
Here, the gamma-characteristic is expressed thus: EQU output=(input).sup..gamma. ( 1)
From Equation (1), the output of the gamma-correction circuit is an exponential function of the exponent gamma with respect to the input thereof. That is, if the resolution of output signal is composed of eight bits, the resolution of the input signal should have ten or more bits. Accordingly, in the conventional video camera, the gamma-correction circuit should have the components of ten or more bits of resolution, which results in a higher cost of the manufactured goods.